The field of the invention is magnetic resonance imaging and systems. More particularly, the invention relates to a method for producing MR images having an image contrast provided by rotary saturation of MR signals that is induced by the application of mechanical waves to a subject.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mxy. A signal is emitted by the excited nuclei or “spins”, after the excitation signal B1 is terminated, and this signal may be received and processed to form an image.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The measurement cycle used to acquire each MR signal is performed under the direction of a pulse sequence produced by a pulse sequencer. Clinically available MRI systems store a library of such pulse sequences that can be prescribed to meet the needs of many different clinical applications. Research MRI systems include a library of clinically proven pulse sequences and they also enable the development of new pulse sequences.
The MR signals acquired with an MRI system are signal samples of the subject of the examination in Fourier space, or what is often referred to in the art as “k-space”. Each MR measurement cycle, or pulse sequence, typically samples a portion of k-space along a sampling trajectory characteristic of that pulse sequence. Most pulse sequences sample k-space in a raster scan-like pattern sometimes referred to as a “spin-warp”, a “Fourier”, a “rectilinear”, or a “Cartesian” scan. The spin-warp scan technique employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of MR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (“2DFT”), for example, spatial information is encoded in one direction by applying a phase encoding gradient, Gy, along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient, Gx, in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse, Gy, is incremented, ΔGy, in the sequence of measurement cycles, or “views” that are acquired during the scan to produce a set of k-space MR data from which an entire image can be reconstructed.
There are many other k-space sampling patterns used by MRI systems These include “radial”, or “projection reconstruction” scans in which k-space is sampled as a set of radial sampling trajectories extending from the center of k-space. The pulse sequences for a radial scan are characterized by the lack of a phase encoding gradient and the presence of a readout gradient that changes direction from one pulse sequence view to the next. There are also many k-space sampling methods that are closely related to the radial scan and that sample along a curved k-space sampling trajectory rather than the straight line radial trajectory.
An image is reconstructed from the acquired k-space data by transforming the k-space data set to an image space data set. There are many different methods for performing this task and the method used is often determined by the technique used to acquire the k-space data. With a Cartesian grid of k-space data that results from a 2D or 3D spin-warp acquisition, for example, the most common reconstruction method used is an inverse Fourier transformation (“2DFT” or “3DFT”) along each of the 2 or 3 axes of the data set. With a radial k-space data set and its variations, the most common reconstruction method includes “regridding” the k-space samples to create a Cartesian grid of k-space samples and then perform a 2DFT or 3DFT on the regridded k-space data set. In the alternative, a radial k-space data set can also be transformed to Radon space by performing a 1DFT of each radial projection view and then transforming the Radon space data set to image space by performing a filtered backprojection.
In recent years, MRI has been increasingly employed to guide interventional medical treatment procedures. Exemplary procedures include MR-guided focused ultrasound, biopsies, cryoablation, and laser, radiofrequency (“RF”), or microwave thermal ablation. In these MR-guided procedures, the MRI system produces images that depict the anatomy of the subject being treated and also the location of the medical instrument used to perform the treatment. Additionally, other images such as temperature maps that indicate the amount of thermal energy imparted to tissues during, for example, an RF ablation procedure, can be produced. These anatomical and other images are produced in real-time as treatment is performed, thereby assisting the physician in physically guiding the instrument into proper position.
Magnetic resonance guided high intensity focused ultrasound (“MRgFUS”) ablation is an attractive, non-invasive method that selectively ablates deep-lying tissue. The therapeutic value of this alternative surgical technique, however, depends on the accuracy of the imaging methods utilized to monitor the deposition of thermal energy to the target and surrounding tissues. There are several MR imaging techniques for measuring temperature change, including those that use T1-weighted imaging, diffusion weighted imaging, and water proton resonance frequency (“WPRF”) imaging based methods. Among these different temperature-imaging methods, WPRF temperature imaging is the commonly preferred technique for MRgFUS ablation therapy. In WPRF temperature imaging, temperature changes are calculated from phase difference images, usually acquired using a fast gradient echo sequence performed before and after FUS sonication. Several studies have demonstrated that WPRF temperature imaging can be used to accurately monitor temperature change in vivo in muscle tissue during MRgFUS ablation treatment.
While it is useful to monitor temperature changes in regions that have already been heated during MRgFUS ablation treatment, a method that allows the direct visualization of the ultrasound focus before heating would be similarly valuable. For example, in some applications, such as FUS treatment of the brain, the array of ultrasound sources must be properly tuned. However, this process is difficult due to phase shifts in the FUS beam that are induced by the skull. Therefore, it would be highly advantageous for the clinician to be able to determine the quality of the FUS beam focus in a target region non-invasively, and to do so at low power. Additional uses for MRgFUS include opening the blood-brain-barrier for the delivery of pharmaceutical agents to brain tissues. Such methods hold promise in gene therapy treatments. In these applications it would be likewise advantageous to non-invasively determine the focus of the FUS beam at a low power and prior to performing the treatment procedure. When performing MRgFUS on the brain, it is important to ensure that the focus of the FUS beam is properly set, so that thermal energy is not imparted to unwanted regions of healthy tissue or critical brain structures. Therefore, it is important to be able to properly and accurately identify the regions affected by the sonication.
It has been found that MR imaging can be enhanced when an oscillating stress is applied to the object being imaged in a method called MR elastography (“MRE”). The method requires that the oscillating stress produce shear waves that propagate through the organ, or tissues to be imaged. These shear waves alter the phase of the MR signals, and from this, the mechanical properties of the subject can be determined. In many applications, the production of shear waves in the tissues is a matter of physically vibrating the surface of the subject with an electromechanical device. The gradients employed in MRE must be modulated at the same frequency as the applied stress and the rate at which magnetic gradient fields can be modulated is limited. This limits the practical use of MRE to the application of low frequency oscillatory stresses, which are of little use for imaging the effects of the higher frequencies present in FUS procedures.
It would therefore be desirable to provide a method for non-invasively imaging the presence of a mechanical oscillation imparted to a subject, in which the mechanical oscillation has a relatively high frequency. Such a method would be applicable to image the effects of magnetic resonance guided focused ultrasound (“MRgFUS”) procedures, and could specifically be employed to determine the accuracy of the FUS ultrasound beam focus in a target region.